The present invention generally relates to radiocommunication systems and, more specifically to an apparatus and associated method for efficiently reducing the number of battery cells within mobile units (e.g. dual band phones) without increasing noise or spurs.
Voltage controlled oscillators (VCOs) are widely used in communication devices such as cellular telephones to generate oscillatory signals. In particular, a voltage controlled oscillator is one of the components of a phase-locked loop or PLL, which is an electronic circuit used to generate a stable oscillatory signal having a desired frequency of oscillation. Such circuits are commonly referred to as frequency synthesizers. The signal output by a PLL frequency synthesizer may be used, for example, as a carrier signal in a transmitter or as a local oscillator signal in a receiver.
In order to tune a transmitter or receiver to a particular frequency, it is necessary to alter the frequency of oscillation of the carrier or local oscillator signal in a controlled manner. This can be achieved by synthesizing the carrier and local oscillator signals using a programmable frequency synthesizer including a VCO. A VCO is tuned by varying an input DC control voltage level.
Most portable electronic devices use a five-cell or four-cell battery to generate DC power. Devices having five-cell and four-cell batteries can generate a regulated DC power supply voltage level of about 4.8 volts and 3.8 volts, respectively. In the case of a five-cell battery, the DC voltage control signal, V.sub.C, used as a control input to the VCO has a range of between about 0.7 and 4.3 volts, or approximately 3.6 volts. In the case of a four-cell battery, V.sub.C has a range of between about 0.6 and 3.0 volts, or 2.4 volts. The measure of the frequency responsiveness of a VCO with respect to input control voltage V.sub.C is known as the gain of the VCO, and is measured in megahertz per volt (MHz/V). Thus, if the input control voltage has a lower range, the VCO should have a greater gain in order to be tunable over the same frequency range.
To be useful in an RF communication device such as a dual band cellular telephone, a voltage controlled oscillator should be tunable over a range of approximately 90 MHz for some applications. Thus, a VCO for use in a device having a five-cell supply has a gain of about 26 MHz/Volt. A VCO for use in a device having a four-cell supply has a gain of about 40 MHz/Volt.
One drawback to a VCO is the fact that it produces a certain amount of phase noise, which can degrade the performance of a communications system. A VCO must be designed such that phase noise is kept within certain predetermined limits. In a voltage controlled oscillator, phase noise follows two rules. The first is that phase noise is directly proportional to the gain of the oscillator. The second is that the phase noise is inversely proportional to the magnitude of the control voltage, which is applied across the varactor. Both rules are based upon the fact that the varactor diode in a VCO exhibits an internal resistance that is greater when a low control voltage range is applied across its terminals. This greater internal resistance decreases the quality factor of the oscillator's resonant circuit leading to increased phase noise. The former rule simply implies that the greater the gain of the oscillator, the greater the varactor diode's contribution to the quality factor of the resonant circuit. Thus a high gain VCO with a low control voltage range would exhibit the poorest phase noise characteristics.
There is currently a trend in the communication industry to design and produce electronic devices capable of operating with a three-cell battery. A three-cell battery provides a control voltage range of from about 0.5 to 2.2 volts, or 1.7 volts. A conventional VCO having a gain corresponding to such a control voltage range would exhibit unacceptable phase noise characteristics. A VCO used in each band of a dual band phone system would solve the voltage requirement problem, however, two VCOs require too much space and are expensive. Also, a switch VCO tank circuit could also be used to remedy the reduction in batteries. However, the use of the switch increases standby time in any band which draws current. Thus, there is a need for a voltage controlled oscillator and related control circuitry which exhibits acceptable phase noise performance in a device having a low supply voltage level.